Interrelationship of hexosaminidases A and B: confirmation of the common and the unique subunit theory

Human kidney hexosaminidase A (β N acetylglucosaminidase; 2 acetamido 2 deoxy β D glucoside acetamidodeoxyglucohydrolase; EC 3.2.1.30) is a heteropolymer of two immunologically distinct subunits designated as α and β. Hexosaminidase B, however, is a homopolymer comprised entirely of β subunits. When human kidney hexosaminidase A was dissociated into its subunits by p hydroxymercuribenzoate, three distinct proteins having isoelectric points of pH 7.2, 5.4, and 4.3 were isolated. The fraction having an isoelectric point of pH 7.2 designated as β fraction, was electrophoretically and immunologically identical to hexosaminidase B and was enzymatically active. The proteins having isoelectric points of pH 5.4 and 4.3, designated as hexosaminidase Ai and α fractions, respectively, were enzymatically inactive and crossreacted with antiserum against hexosaminidase A and not with antiserum against hexosaminidase B. Upon incubation of p hydroxymercuribenzoate treated hexosaminidase A with dithiothreitol, hexosaminidase A activity, as well as antigenicity, was regenerated, indicating that α and β subunits hybridize to form hexosaminidase A. Antibodies raised in rabbits against β fractions reacted with both hexosaminidase A and B, whereas the antibodies against α and hexosaminidase Ai fractions reacted only against hexosaminidase A. This would indicate that both fractions are composed only of subunits unique to hexosaminidase A. The molecular weights of α, β, and hexosaminidase Ai fractions were estimated to be 47,000, 120,000, and 180,000 respectively, by Sephadex gel filtration. Upon urea sodium dodecyl sulfate polyacrylamide electrophoresis, each of the three fractions dissociated into a single polypeptide having a molecular weight of approximately 18,000. It is concluded that p hydroxymercuribenzoate dissociates hexosaminidase A, (αβ)₃, into its subunits, and the β subunits can reassociate to form relatively stable hexosaminidase B, (ββ)₃, while the α subunits reassociate in both the dimeric state, α₂, and a polymeric state, α₈.